In a typical Wireless Local Access Network (WLAN) network, all devices contend for accessing the wireless medium. The basic Medium Access Control (MAC) mode of contention is based on the Carrier Sensing Multiple Access (CSMA) mechanism. In order to enhance the performance of CSMA over the wireless medium, some modifications have been proposed. For instance, the original 802.11 standard (Rev1997) specifies CSMA with collision avoidance (CSMA/CA) and lately the 802.11e amendment specifies enhancements to provide a better Quality of Service (QoS).
One of the enhancements introduced by 802.11e is the concept of a transmission opportunity (TxOP). During a TxOP, multiple packets can be transmitted during the granted time. The TxOP has proven to be a very efficient enhancement to the basic MAC. The main idea with the introduction of a TxOP is to limit the overall amount of time a Station (STA) can spend on a channel once it has won contention. Before the advent of 802.11e, a STA, once it won the contention-based access, could transmit as long as it had data to transmit. This led to situations where a particular STA could basically pre-empt the wireless medium since it had so much data to transmit, resulting in detrimental side-effects to traffic streams by other STAs in the Basic Service Set (BSS). In order to remedy this problem, 802.11e introduced TxOPs with the idea that no STA could occupy the wireless medium longer than the TxOP length, and therefore, the medium would be opened up for contention by all STAs again at a minimum guaranteed rate.
However, sometimes a station may have no more data to transmit over the whole TxOP and therefore the bandwidth could be wasted. For these cases, the 802.11e amendment provides a mechanism to relinquish the medium so that other devices can use this previously granted time, and contend again for the medium. Here, a Contention Free (CF)-End frame, which can be sent only by an Access Point (AP), is used to reset the Network Allocation Vector (NAV) for all stations in the system and to communicate to the BSS the fact that contention could start again, even if it is prior to the expiration of the original TxOP. The 802.11n enhances this concept further by allowing any STA to truncate its TxOP with a CF-End.
Currently, the 802.11n group is working on further enhancements to the standard in order to provide for higher throughput. One of these enhancements is called Reverse Direction (RD). In conjunction with this, the 802.11e TxOP may be over-provisioned for a reverse-direction transmission in order to increase medium efficiency by reducing the number of medium-access attempts. The RD concept introduces a different usage for the remaining unused/over-provisioned time in a TxOP. Instead of relinquishing the medium to all stations to contend again, it specifically allows the peer station (i.e. TxOP receiver) to reuse the remaining time in a TxOP to send data on the reverse direction link to the originator.
One advantage of such Reverse Direction Grants (RDG) is that the contention for the medium by the peer station, which is time costly, does not take place anymore and overall relative medium-occupancy (ratio of data transmission to contention time per time period) is increased. Another advantage is the resulting reduction in latency for the reverse direction transmission (which might have otherwise been delayed by medium access contention), which is particularly useful in relatively symmetric real-time traffic scenarios such as VoIP.
In a WLAN mesh system, a set of two or more Mesh Point (MPs) are interconnected via IEEE 802.11 links. Each MP on a mesh network receives and transmits its own traffic, while acting as a router or forwarder for other nodes. Mesh networks are also known as multi-hop networks, since packets might be relayed more than once in order to reach their destination.
Accordingly, this represents a different paradigm as compared to the original WLAN standard, which addressed only star topologies (e.g. BSS, IBSS) and therefore single hop communications.
One specific problem occurring when adopting the current 802.11n RDG method to a WLAN Mesh context is that channel access contention-resolution (or deterministic) allocation is different than in a BSS context (like assumed in 802.11n) where all STAs are guaranteed to be in communication range with the AP. In a WLAN Mesh network, either node on one side of a particular Mesh link is in communication range with only a subset of other Mesh nodes. However, winning a TxOP by one of the nodes needs to pre-empt all other nodes in interference range from transmitting during this TxOP. Even if both nodes on the link can re-use the existing 802.11n RDG protocol to respectively arbitrate the use of a particular TxOP amongst themselves, the current mechanism cannot guarantee collision free communication in-between the pair, since no method exists to communicate the changed usage of the granted TxOP to at least the tier-1 neighbors.
An additional practical design problem currently not addressed by the original 802.11n RDG method is a practical question arising from WLAN Mesh network design where traffic is not carried back-and-forth across links of multiple Mesh nodes (but only between a particular pair of nodes in a BSS). There are many usage scenarios in which there can be substantially more gain in terms of node throughput and traffic latency when the remainder of a TxOP is used by a MP on a Mesh link for carrying received traffic forward (rather than using it to send traffic back to the originating MP in the reverse direction). It should be noted that this problem cannot arise in a traditional infra-structure type BSS, where any traffic is only carried back-and-forth in between a particular pair of devices, i.e. from/to the AP to/from a given STA.
Therefore, there exists a need for a method that realizes the gain with the 802.11n RDG method for WLAN mesh networks, that is not subject to the limitations of the existing art. There also exists a need for a method that improves upon the idea of efficiently re-using remaining TxOP time by MPs for WLAN Mesh networks.